With a rapidly aging population, diseases of the vascular system are of great concern to our society. Arterial thrombosis is the major cause of death in the form of heart attacks and strokes,while venous thrombosis is associated with pulmonary embolism which occurs after surgery or extended periods of inactivity.
Thrombin is a multifunctional serine protease whose role in thrombosis and hemostasis has been documented by a number of sources (See generally, Tapparelli, et al. TiPS 1993, 14, 366-76). Thrombin acts as a procoagulant through proteolytic cleavage of fibrinogen to form fibrin and as an anticoagulant through activation of the protein C pathway. (followed by inactivation of coagulation factors V and VIII.) The concentration of active thrombin is limited by a number of feedback mechanisms involving endogenous factors and proteins. In addition to protein C, antithrombin III is another regulating protein which forms a complex with endogenous heparin. This complex binds to active thrombin, thus inactivating it.
Current anticoagulant therapy consists of three classes of compounds: heparins, coumarins and low molecular weight heparins. These drugs act indirectly to limit the concentration of active thrombin. Heparins and low molecular weight heparins interact with antithrombin III and the coumarins inhibit a number of vitamin K dependent coagulation factors. Although these drugs are prescribed for diseases associated with venous thrombosis and arterial thrombosis, their use is limited. They have a number of side effects, a slow onset of action and only the coumarins are orally active (warfarin and dicumarol).
Indirect thrombin inhibitors have been shown to be less effective at controlling associated diseases than direct thrombin inhibitors. Thus the search for orally active direct thrombin inhibitors is underway in a number of laboratories.
These efforts have produced a number of small peptidyl compounds which directly inhibit thrombin. PPACK, argatroban, (D)-NAPAP and DUP 714 are examples of the direct thrombin inhibitors of interest. ##STR2##
Unfortunately, only some of these compounds are weakly orally active and most have a poor selectivity for thrombin versus other serine proteases. Therefore, a need remains for direct thrombin inhibitors which exhibit good selectivity over other serine proteases and are orally active.